Adult Plant Slow Rusting Genes Confer High Levels of Resistance to Rusts in Bread Wheat Cultivars From Mexico.

Rust diseases continuously threaten global wheat production: stem rust, leaf rust, and yellow rust caused by Puccinia graminis f. sp. tritici, Puccinia triticina, and Puccinia striiformis f. sp. tritici, respectively. Recent studies indicated that the average losses from all these three rusts reached up to 15.04 million tons per year, which is equivalent to an annual average loss of around US $2.9 billion per year. The major focus of Mexican and worldwide breeding programs is the release of rust resistant cultivars, as this is considered the best option for controlling rust diseases. In Mexico, the emphasis has been placed on genes that confer partial resistance in the adult plant stage and against a broad spectrum of rust races since the 1970s. In this study, a set of the first-generation tall varieties developed and released in the 1940s and 1950s, the first semi-dwarfs, and other releases in Mexico, all of which showed different levels of rust resistance have been phenotyped for the three rust diseases and genotyped. Results of the molecular marker detection indicated that Lr34, Lr46, Lr67, and Lr68 alone or in different gene combinations were present among the wheat cultivars. Flag leaf tip necrosis was present in all cultivars and most were positive for brown necrosis or Pseudo Black Chaff associated with the Sr2 stem rust resistance complex. The phenotypic responses to the different rust infections indicate the presence of additional slow rusting and race-specific resistance genes. The study reveals the association of the slow rusting genes with durable resistance to the three rusts including Ug99 in cultivars bred before the green revolution such as Frontera, Supremo 211, Chapingo 48, Yaqui 50, Kentana 52, Bajio 52, Bajio 53, Yaqui 53, Chapingo 53, Yaktana Tardio 54, and Mayo 54 and their descendants after intercrossing and recombination. These slow rusting genes are the backbone of the resistance in the current Mexican germplasm.

Regional differences in the composition of Fusarium Head Blight pathogens and mycotoxins associated with wheat in Mexico.

Fusarium Head Blight (FHB) is a destructive disease of small grain cereals and a major food safety concern. Epidemics result in substantial yield losses, reduction in crop quality, and contamination of grains with trichothecenes and other mycotoxins. A number of different fusaria can cause FHB, and there are significant regional differences in the occurrence and prevalence of FHB pathogen species and their associated mycotoxins. Information on FHB pathogen and mycotoxin diversity in Mexico has been extremely limited, but is needed to improve disease and mycotoxin control efforts. To address this, we used a combination of DNA sequence-based methods and in-vitro toxin analyses to characterize FHB isolates collected from symptomatic wheat in Mexico during the 2013 and 2014 growing seasons. Among 116 Fusarium isolates, we identified five species complexes including nine named Fusarium species and 30 isolates representing unnamed or potentially novel species. Significant regional differences (P < 0.001) in pathogen composition were observed, with F. boothii accounting for >90% of isolates from the Mixteca region in southern Mexico, whereas F. avenaceum and related members of the F. tricinctum species complex (FTSC) accounted for nearly 75% of isolates from the Highlands region in Central Mexico. F. graminearum, which is the dominant FHB pathogen in other parts of North America, was not present among the isolates from Mexico. F. boothii isolates had the 15-acetyldeoxynivalenol toxin type, and some of the minor FHB species produced trichothecenes, such as nivalenol, T-2 toxin and diacetoxyscirpenol. None of the FTSC isolates tested was able to produce trichothecenes, but many produced chlamydosporol and enniatin B.